Like all testing methods, ultrasound inspection is also standardized and is carried out in accordance with guidelines, such as according to DIN EN 10228-3 1998-07 Zerstörungsfreie Prüfung von Schmiedestücken aus Stahl—Teil 3: Ultraschallprüfung von Schmiedestücken aus ferritischem and martensitischem Stahl (Non-destructive testing of steel forgings—Part 3: Ultrasonic testing of ferritic or martensitic steel forgings), which is hereby incorporated by reference. Suitable testing devices and methods are known for the non-destructive testing of a workpiece by means of ultrasound. General reference is made to the textbook by J. and H. Krautkrämer, Werkstoffprüfung mit Ultraschall, ISBN-13: 978-3-540-15754-0, 5th edition (1986), Springer (Berlin).
These methods are generally based on the reflection of sound on boundary surfaces. The sound source most frequently used is an ultrasonic probe or test probe whose radiation is in the frequency range of 10 kHz to 100 MHz. In the case of the pulse-echo method, the ultrasonic probe does not emit a continuous radiation, but very short sound pulses with a duration of 1 μs and less. The pulse emanating from the transmitter passes through the workpiece to be inspected with the respective speed of sound, and is almost completely reflected at the solid-air boundary surface. The sound probe is most frequently not only able to transmit pulses, but also to convert incoming pulses into electrical measuring signals; it thus also works as a receiver. The time required by the sound pulse to travel from the transmitter through the work piece and back again is measured with an oscilloscope or a computer unit. Given a known speed of sound c in the material, the thickness of a workpiece, for example, can thus be checked. The core of such a probe is at least one ultrasonic transducer, e.g. in the form of a piezo-electric element. Furthermore, it is known, for example from WO 2007/144271, to use a phased array of several, separately controllable ultrasonic transducers that are in a fixed spatial relation for the generation and reception of the ultrasonic pulses.
The transducer(s) are most frequently coupled to the workpiece to be inspected with a matching layer—also referred to as leading body—disposed therebetween and having, for example, a wedge shape and most frequently being made of a thermoplastic synthetic material such as poly(methyl methacrylate) (PMMA). A coupling surface is provided on the leading body via which the sound generated by the ultrasonic transducer(s) can be coupled into the workpiece to be inspected, with the wedge shape causing the sound to enter into the workpiece obliquely. The leading body and the piezo-electric element(s) are generally disposed in a housing which is closed on its one side and which, on its other side, has a coupling aperture through which the ultrasound emitted by the sound coupling surface can exit.
In order to couple the workpiece and the probe, i.e. the leading body, a couplant (e.g. a glue (solution), gel, water or oil) is applied onto the surface of the workpiece to be inspected. The surface to be tested is most frequently passed over with the probe. This can take place manually, mechanically or automatically (for example in production lines). In the latter case, the test piece is often immersed in a suitable liquid (immersion technique) or wetted in a defined manner for the purpose of transmitting the sound signal.
The sound velocity is one of the essential quantities by means of which the exact location of flaws of a workpiece and/or its dimensions are determined. The sound velocity is assumed to be known in most ultrasound methods.
However, it was found that the sound velocity of the thermoplastic material used for the leading body is highly temperature-dependent, at least in comparison with the primarily metallic workpieces that are to be inspected. This problem is already known from DE 3327526 A1. Here, for temperature compensation in a TR probe, the travel time and thus the temperature-dependent sound velocity is determined by means of a test reflector fitted into the leading body in order to compensate travel time deviations.
A change of the sound velocity in the leading body due to a temperature change, for example in a wedge-shaped leading body, can lead to the insonification angle into the workpiece changing, for example, in comparison with the specification on the leading body. This may also lead to errors also in the case of an electronic adjustment of the insonification angle, for example by means of a phased array.
Furthermore, the determination of sound velocities in a workpiece using the back-face echo of the workpiece is known from DE 3441894 A1.